Electrical circuitry

ABSTRACT

The invention describes a fail safe voltage responsive circuit for producing an output level indicative of the relationship between two voltage levels. The voltage responsive circuit includes a fail-safe comparison circuit which assumes alternative modes in response to an alternating signal introduced into the comparison circuit when the voltage levels are within a predetermined range, one relative to the other. When the voltage levels are outside the range or if a component fails, the comparison circuit ceases to alternate between modes. The voltage responsive circuit is used in a preferred application involving a fail-safe combustion control system wherein one voltage level is used to supply bias current to a photoconductive cell, the cell sensing the presence or absence of flame.

United States Patent 1 [111 3,710,149 Thomson [4 1 Jan. 9, 1973 [75] Inventor: Elihu Craig Thomson, Wellesley,

3,033,996 5/1962 Atherton [54] ELECTRICAL CIRCUITRY Primary Examiner-Herman Karl Saalbach Assistant Examiner-B. P. Davis Mass Attorney-Willis M. Ertman [73] Assignee: Electronics Corporation of America,

Cambridge, Mass. [57] ABSTRACT Filed: J 1971 The invention describes a fail safe voltage responsive [21] APPL No: 155,062 circuit for producing an output level indicative of the relationship between two voltage levels. The voltage responsive circuit includes a fail-safe comparison cirl CL cuit which assumes alternative modes in response to 250/217 an alternating signal introduced into the comparison hit. Cl. circuit when the voltage levels are within a predeter- Field of Search 307/310, 311, 235, mined range, one relative to the other. When the volt- 115, 6 age levels are outside the range or if a component fails, the comparison circuit ceases to alternate References Cited between modes. The voltage responsive circuit is used UNITED STATES PATENTS ibn a preferred application involving a fail-safe cornust1on control system wherein one voltage level 1s 3,492,589 1/1970 Rotier ..328/ 146 used to supply bias current to a photoconductive cell,

---- 146 the cell sensing the presence or absence of flame.

3,l78,583 4/1965 Koch 3,601,614 8/1971 Platzer ..307/311 10 Claims,4Drawing Figures 22K max mu 2.2K

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ELECTRICAL CIRCUITRY SUMMARY OF INVENTION The invention relates to improved control apparatus and more particularly to means for insuring fail safe operation of a voltage responsive system when two voltages are outside of a predetermined range one relative to the other.

In condition responsive control systems of the type employed for the supervision of flame in a combustion chamber, the system arrangement must reliably and accurately indicate the presence or absence of flame, the absence of flame being promptly detected so that the fuel valve may be quickly closed thus preventing an excessive amount of unburned fuel from accumulating in the combustion chamber. Malfunction of a component of such a system may cause the flame detecting system to falsely indicate the presence of flame and thus create and unsafe condition in the event of flame failure as the system would continue to react as if flame were present.

A photoconductive cell may be used in such a system. Use of a photoconductive cell requires a DC voltage source to provide the initial bias current, said current being modulated by the changes in conductivity of the photoconductive device due to irradiation. The fluctuations in bias current, typically at 101-12 when flame is present, are detected by the control system circuitry. Such a cell may have a substantial steady-state conductivity even when not irradiated, in which case it is necessary to insure that the source of bias current is itself steady and cannot vary at a rate corresponding to the rate of fluctuation of the signal, thus giving a false indication of flame.

It is an object of this invention to provide a novel circuit for monitoring the bias voltage in a combustion control circuit in which a component will fail only in a safe direction.

Another object of this invention is to provide a novel comparison circuit which produces an output signal indicating when the relative values of two voltages are within a predetermined range and wherein a circuit component failure is in the safe direction.

It is a further object of the invention to provide a novel and improved voltage comparison circuit which is simple, inexpensive, and fail-safe.

The invention concerns a voltage responsive system which produces an output signal indicative of a predetermined condition between two voltages. The system includes a voltage comparison circuit and the invention features a source of alternating voltage of predetermined magnitude which enables the system to operate in an operative mode, where the comparison circuit in response to the source switches between alternate states when the two voltages being compared are within a predetermined range determined by the magnitude of the alternating voltage source, and a second inoperative mode where the circuit assumes a single state when the first and second voltages are outside the predetermined range. An output signal, produced by the circuit, has a substantial alternating component in the operative mode and a negligible alternating component in the inoperative mode.

In particular embodiments, the output signal charges a fail-safe capacitor-diode circuit to a substantial voltage signal level in the operative mode and to a negligible voltage signal level in the inoperative mode. The comparison circuit assumes the inoperative mode upon failure of a single component.

In a particular embodiment the comparison circuit features a first transistor and the source of alternating voltage appears as an additive component to said second voltage at the base of the transistor. A shunt regulator voltage from a zener diode is connected to the emitter of the transistor and the output signal is derived from the collector of the transistor. The transistor, in the operative mode, is alternately conductive and cut off. The base voltage divider resistors are more resistive than the collector resistor, which is larger than the voltage dropping resistor associated with the zener diode.

In a second particular embodiment, a second transistor has its base connected through a resistor to the collector of the first transistor.

In still another embodiment, the voltage comparison circuit is arranged to assume a first operative mode comprising one state when the voltages are within a predetermined range relative to one another and a second operative mode comprising two states when the voltages are outside the predetermined range. The voltage comparison circuit alternately assumes the first mode and the second mode in response to the alternating voltage when the two voltages are within a second predetermined range one relative to the other. A signal having an alternating component is produced when the voltages are with the second predetermined range. The comparison circuit assumes only one mode when the two are outside the second predetermined range and assumes a single mode in response to a single component failure. This voltage comparison circuit comprises two transistors connected with a common emitter load resistor. The alternating voltage is applied additively through the base of the first transistor and third and fourth voltages substantially proportional respectively to the first two voltages are applied to the bases of the two transistors, respectively.

DESCRIPTION OF PARTICULAR EMBODIMENTS Referring to FIG. 1, there is shown a simple circuit diagram embodying the invention. A shunt regulator, here zener diode 10 with voltage dropping resistor 12 are connected to the emitter 14 of NPN transistor 16. Resistive load 17 is connected across the zener diode. Resistors l8 and 20 are proportioned so that to DC component of voltage at the base of transistor 16 is approximately the same as the voltage drop across zener diode 10. Collector resistor 22, connected to the collector of transistor 16, provides current from positive power supply bus 24.

A small alternating voltage is introduced at the lower end of and is series with resistor 20. The source of the alternating voltage may be the secondary of a transformer 26, for example a low voltage tap from a power transformer as shown here. This alternating voltage is just sufficient to cause the base of transistor 16 to alternately rise above its quiescent DC value and cause the transistor to saturate when the additive alternating voltage is positive and to fall below its quiescent DC value ensuring that the transistor is cut-off when the alternating voltage is negative. Thus, at the collector of transistor 16, a voltage signal having an alternating component is produced which, when the transistor is cut-off approaches the power supply voltage and when the transistor is saturated is substantially equal to the voltage of the zener diode. Output circuit 28 converts the alternating collector signal to a negative output level E The alternating component of the collector voltage signal is coupled by capacitor 30 to a rectifier and filter circuit consisting of diodes 32 and 34 and filter capacitor 36. Diodes 32 and 34 are arranged to produce a voltage level across the filter capacitor which is negative with respect to the common bus 38.

With this arrangement a failure of any non-resistive component will cause the voltage across the filter capacitor to decrease to zero.

If the zener diode fails open-circuited, the transistor will be cut-off, the emitter voltage rising above the zener held voltage, and the collector will remain at a constant voltage level. The coupling capacitor 30 effectively blocks the constant voltage component and the filter capacitor discharges into the load 40, the voltage across the filter capacitor going to zero. Short circuit of the zener causes the transistor to always be saturated, and the collector voltage thereby being constant, the output voltage again reduces to zero.

Short circuit failure of the resistors 12, 18, 20 or 22 is considered to be impossible if the resistor is used at not more than 30% of its rated power. An open of resistor 20 will prevent the alternating component of voltage from affecting the transistor and therefore the filter capacitor voltage will disappear. Open circuit failure of resistor 18 causes the transistor to cut-off thereby blocking the effect of the alternating voltage and causing the output signal to disappear. An open circuit of resistor 22 cuts off collector current thereby cutting off the alternating signal at the transistor collector. An open circuit of resistor 12 for some component values not affect the output voltage E,,,,. The resistance of resistor'22 is chosen to be much larger, here fifty times larger, than the resistance of resistor 12. Therefore if resistor 12 opens, the emitter current through load 17 is insufficient to raise the emitter voltage to a level where the transistor will cut off. Therefore the transistor saturates and the alternating signal is blocked.

In the transistor, any open circuit will either cut off the alternating signal directly or will cut off a bias current essential for transmission of the alternating component. A collector to emitter short circuit occurs, the zener voltage controls the voltage level at the collector; and the zener holds the collector voltage level constant, E thus disappearing. E also disappears as a result of a base to emitter short circuit transistor 16 thereby being cut off and the collector voltage remaining constant. A base to collector short circuit may, under certain circumstances allow the alternating component to continue to produce an output lever E If, however, resistors 18, 20 are chosen-significantly larger than resistor 22, as shown in FIG. 1, the collector to base short tends to raise the voltage level at the base of transistor 16 which is however held constant by the action of zener through the base-emitter diode. The voltage level at the base and therefore the collector is held constant even though the voltage at the lower end of resistor varies. Capacitor 36 thereby discharges through the load and E disappears.

The requirement that resistor 22 be large in comrent to be drawn from capacitor 36, the charging current, determined by resistor 22 being small. Thus, the load willhere be greater than for example, kilohms. FIG. 2 shows the preferred embodiment according to the invention, an elaboration of the circuit of FIG. 1 in which resistor 22 is eliminated and PNP transistor 50 is added to allow current to be drawn from the output circuit. The collector current of transistor 16 flows through the base-emitter junction of transistor 50 through resistor 51, causing transistor 50 to be completely conductive when current is flowing in transistor 16, and cut off when current in transistor 16 is not flowing.

The alternating voltage supplied to the rectifier is then obtained across collector resistor 52. In this embodiment, the voltage intermittently appearing across resistor 52 is the maximum possible--that of the main voltage supply itself. Transistor 50 provides the current gain necessary to allow power to be drawn from the output circuit 28 and also allows the resistance value of resistors 18, 20 to be reduced.

The added transistor 50 also fails safely, for any open circuit or a base-emitter short will cut the transistor off so as to remove signal to resistor 52, while a collectorbase or collector-emitter short will place the supply voltage continuously across resistor 52, removing any AC signal in both conditions.

FIG. 3 shows the preferred application of the failsafe zener monitoring circuit of FIG. 2, a complete failsafe combustion control system. In this arrangement, the voltage across capacitor 36 provides the supply voltage for one of the stages of amplification of the flame detection circuit. Failure of this supply voltage to the collector of transistor 54 through resistor 56 results in transistor 54' ceasing to operate as an amplifier, so that no signal can pass, other than that coupled directly to the collector which is negligible. This circuit shows small variation of the voltage-regulator monitor circuit in which the supply voltage for the monitor circuit is derived from a second zener diode 58, fed from a voltage-dropping resistor 59. In this case, the monitor circuit will also monitor zener diode 58, in that if zener 58 fails open the supply voltage will rise, and consequently the base voltage of transistor 16, which is derived from this supply through resistors 18 and 20, will also rise. Therefore, transistor 16 will saturate and the AC signal will disappear, as will the voltage across capacitor 36. In this embodiment of the monitor circuit, the AC signal is derived from the power supply transformer 61 by voltage-dropping resistor 62. Although this resistor provides a path for DC which shunts resistor 20, the value of resistor 62 is too high to upset the DC relationship and in any case the value of resistor 20 can be adjusted to allow for the shunt path.

Operation of the complete circuit of FIG. 3 is as follows. The DC power supply voltage is dropped to a first lower voltage by zener diode 58 and then to a second still'lower voltage by zener diode 10 so that resistors 63 and 64 are supplied by a doubly-regulated voltage. Photoconductive cell 65, for example lead sulfide, has a dark resistance which is reduced when it is irradiated by infrared radiation. When exposed to flame, the infrared radiation from the flame will fluctuate, the fluctuations having a main frequency in the vicinity of Hz. Transistor 66 is an amplifier stage, self-biased from its collector. The AC signal is coupled by capacitor 67, which is sized so as not to pass frequencies substantially lower than 10 Hz. Capacitors 68, 69 are sized to feed back degeneratively frequencies substantially higher than 10 Hz. Therefore, transistor 66 iwth its associated passive components, constitutes a band-pass amplifier stage. Transistor 54 operates in a similar manner, except that it is in the PNP configuration so that it will operate from the negative supply voltage derived from the fail-safe monitor circuit. Transistor 74 also operates in a similar manner, except that it is driven by transistors 75, 76 arranged in a Darlington configuration, whose collector 77 is biased to half the supply voltage by a resistive divider consisting of resistors 79,

80. Transistor 82 is a bootstrap emitter-follower used as the load for transistor 74, so that when transistor 74 is conducting, transistor 82 is cut off, and the discharging current for capacitor 84 comes from transistor 74. When transistor 74 is cut off, transistor 82 is biased to conductivity through resistor 86 so that is supplies the charging current for capacitor 84. The result is that transistor 74 is a power AC driver feeding a rectifier comprising diodes 89 and 90, through inductor 91 and capacitor 84. Inductor 91 is proportioned to block frequencies substantially higher than 10 Hz. The combination of inductor 91, capacitors 84, 92, and diodes 89, 90 operate as a fail-safe circuit. Thus, the 10 Hz component of signal derived from the flame: is amplified to sufficient power to energize relay 94 when rectified.

Except for short circuits in the resistors and the inductor which are considered to be impossible, all circuit component failures will result in the de-energization of relay 94 even when the power line voltage is fluctuating at the 10 Hz signal frequency so as to modulate the main DC supply voltage at this frequency.

When the monitored voltage source is sensitive to current drain, it may be necessary to add a transistor to substantially isolate the voltage source from the comparison circuit. FIG. 4 shows such an embodiment according to the invention wherein two voltage levels E and E, are compared and an output voltage level E results, the level being negative when the voltages are within a predetermined range relative to one another and disappearing when the signals are outside the predetermined range or when a component fails. The voltage sources may here be current sensitive, each being isolated by a respective transistor. NPN transistors 100 and 102 are connected in a comparison circuit with a common emitter load resistor 104. Resistor dividers 106, 108 and 110, 112 are chosen to produce at the base of the respective transistors 100 and 102, substantially identical quiescent voltages. In one of the base paths the small alternating voltage is injected, here by transformer 114 whose primary is connected to the AC supply voltage. This alternating voltage is just sufficient to cause the base of transistor 102 to be alternately above and below the base of transistor 100. When it is above, the current in resistor 104 is dominated by transistor 102 thereby causing transistor 100 to cut off with no voltage drop in collector resistor 115. When the base of transistor 102 is below the base of transistor 100, transistor 100 dominates the current in resistor 104 results only from transistor 100, and the resulting collector current in transistor produces a voltage drop across resistor 115. The voltage at collector 116 thus changes on alternate half cycles and the alternating component of voltage is coupled through capacitor 30 to the rectifier consisting of diodes 32 and 34 and to filter capacitor 36 producing a negative voltage level E Here also, the failure of a single component results in the disappearance of E Operation with respect to capacitors 30, 36 and diodes 32, 34 has been described in connection with FIG. 1.

Open circuit failure of resistor 104 or resistor 106 causes transistor 100 to be continuously cut off while open circuit failure of resistor 108 causes transistor 100 to continuously saturate both failure modes resulting in a constant voltage level at collector 116 with the resulting disappearance of E Open circuit failure of resistor 110 causes transistor 102 to continuously cut off, the alternating voltage produced through transformer 114 not being sufficient to cause the transistor to turn on, and open circuit failure of resistor 112 causes transistor 102 to saturate, these failures effectively blocking the AC component with E again disappearing. Open circuit failure of either of resistors 104, 115 cuts off the alternating signal by cutting off current to transistor 100. Any open circuit in a transistor will either cut off the alternating signal directly or cut off a bias current essential to the transmission of the signal.

In transistor 100, a short from collector to base will increase the voltage at the base of transistor 100, increasing the voltage at the emitter, thereby causing transistor 102 to cut off so that no alternating signal can appear across resistor 115. A collector-emitter short will cause enough current to flow in resistor 104 to raise the emitter of transistor 102 above the base, causing it to cut off. Resistor 115 must have sufficiently low value to insure that this occurs. A base-emitter short in transistor 100 cuts transistor 100 off so that no current can flow in resistor 115. In transistor 1 02 baseemitter short will cut it off and the alternating signal across resistor 115 ceases; and a collector-emitter or collector-base short will raise the voltage across resistor 104 causing transistor 100 to cut off. Thus any failure of transistors 100, 102 cause E to disappear.

While particular embodiments of the invention have been shown and described, various modifications will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiments or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What is claimed is:

1. In a combustion control system having a photoconductive photocell for sensing flame a regulated voltage source connected in shunt with said photocell and fail-safe circuitry responsive to changes in said photocell for energizing a relay when said flame is sensed, said fail-safe circuitry including at least one stage of amplification, the improvement comprising a fail-safe regulated voltage source monitoring means, said monitoring means comprising a source of standard voltage,

a source of alternating voltage of predetermined magnitude,

said monitoring means being arranged to operate in an operative mode when said regulated and standard voltages are within a predetermined range determined by the magnitude of the alternating current source, and a second inoperative mode when said regulated and standard voltages are outside said predetermined range,

a solid state device having emitter, base and collector electrodes, means to apply said regulated voltage to said emitter electrode,

means including a resistive voltage divider having first and second resistors for applying said standard voltage tosaid base electrode, means for applying a signal from said source of alternating voltage as an additive component to said standard voltage at said base electrode, means for deriving an output signal from said collector electrode, said solid state device being alternately conductive and cutoff, in operative mode and assuming a single state in inoperative mode,

a fail-safe capacitor-diode rectifier circuit connected to the output of said solid state device, said output signal causing said fail-safe rectifier circuit to produce a substantial voltage signal level in said operative .mode and to produce a negligible voltage signal level in said inoperative mode,

and means for applying said voltage signal level to said amplification stage so that said amplification stage is enabled in response to said substantial voltage signal level and disabled in response to said negligible voltage,

said monitoring means assuming said inoperative mode upon failure of any non-resistive component thereof.

2. The system of claim 1 wherein said regulated voltage source includes a zener diode connected in shunt with said photocell, and further including a voltage dropping resistor in series connection with said zener diode.

3. The system of claim 2 wherein said solid state device has a collector resistor supplying bias current to said collector electrode, said resistive voltage divider being more resistive than said collector resistor, and said collector resistor being more resistive than said voltage dropping resistor.

4. The system of claim 1 and further including a transistor having its base electrode connected through a resistor to the collector electrode of said solid state device, and arranged to amplify current in said collector electrode to produce said output signal.

5. The system of claim 1 and further including a transistor having base collector and emitter electrodes and having said regulated voltage connected to its base electrode and the emitter electrode connected to the emitter electrode of said solid state device.

6. The system of claim 5 wherein said regulated and standard voltages are connected to the base electrodes of said solid state device and said transistor through first and second resistive voltage divider circuits, respectively.

7. The system of claim 6 and further including a transistor having its base electrode connected through a resistor to the collector electrode of said solid state device and arranged to amplify current in said collector electrode to produce said output signal.

The system of claim 7 wherein said Solid state device has a collector resistor supplying bias current to said collector electrode, said resistive voltage divider being more resistive than said collector resistor, and

said collector resistor being more resistive than said voltage dropping resistor.

9. The system of claim 8 and further including a transistor having base collector and emitter electrodes and having said regulated voltage connected to its base 

1. In a combustion control system having a photoconductive photocell for sensing flame a regulated voltage source connected in shunt with said photocell and fail-safe circuitry responsive to changes in said photocell for energizing a relay when said flame is sensed, said fail-safe circuitry including at least one stage of amplification, the improvement comprising a fail-safe regulated voltage source monitoring means, said monitoring means comprising a source of standard voltage, a source of alternating voltage of predetermined magnitude, said monitoring means being arranged to operate in an operative mode when said regulated and standard voltages are within a predetermined range determined by the magnitude of the alternating current source, and a second inoperative mode when said regulated and standard voltages are outside said predetermined range, a solid state device having emitter, base and collector electrodes, means to apply said regulated voltage to said emitter electrode, means including a resistive voltage divider having first and second resistors for applying said standard voltage to said base electrode, means for applying a signal from said source of alternating voltage as an additive component to said standard voltage at said base electrode, means for deriving an output signal from said collector electrode, said solid state device being alternately conductive and cutoff, in operative mode and assuming a single state in inoperative mode, a fail-safe capacitor-diode rectifier circuit connected to the output of said solid state device, said output signal causing said fail-safe rectifier circuit to produce a substantial voltage signal level in said operative mode and to produce a negligible voltage signal level in said inoperative mode, and means for applying said voltage signal level to said amplification stage so that said amplification stage is enabled in response to said substantial voltage signal level and disabled in response to said negligible voltage, said monitoring means assuming said inoperative mode upon failure of any non-resistive component thereof.
 2. The system of claim 1 wherein said regulated voltage source includes a zener diode connected in shunt with said photocell, and further including a voltage dropping resistor in series connection with said zener diode.
 3. The system of claim 2 wherein said solid state device has a collector resistor supplying bias current to said collector electrode, said resistive voltage divider being more resistive than said collector resistor, and said collector resistor being more resistive than said voltage dropping resistor.
 4. The system of claim 1 and further including a transistor having its base electrode connected through a resistor to the collector electrode of said solid state device, and arranged to amplify current in said collector electrode to produce said output signal.
 5. The system of claim 1 and further including a transistor having base collector and emitter electrodes and having said regulated voltage connected to its base electrode and the emitter electrode connected to the emitter electrode of said solid state device.
 6. The system of claim 5 wherein said regulated and standard voltages are connected to the base electrodes of said solid state device and said transistor through first and second resistive voltage divider circuits, respectively.
 7. The system of claim 6 and further including a transistor having its base electrode connected through a resistor to the collector electrode of said solid state device and arranged to amplify current in said collector electrode to produce said output signal.
 8. The system of claim 7 wherein said solid state device has a collector resistor supplying bias current to said collector electrode, said resistive voltage divider being more resistive than said collector resistor, and said collector resistor being more resistive than said voltage dropping resistor.
 9. The system of claim 8 and further including a transistor having base collector and emitter electrodes and having said regulated voltage connected to its base electrode and the emitter electrode connected to the emitter electrode of said solid state device.
 10. The system of claim 9 wherein said regulated and standard voltages are connected to the base electrodes of said solid state device and said transistor through first and second resistive voltage divider circuits, respectively. 